1. Field of the Invention
The present invention relates to a beam splitter for use in a light amplification module or the like.
2. Description of the Related Art
In a light amplification module, for example, a part of signal light is taken out as monitor light by a beam splitter, and the power of the monitor light is detected. Then, the signal light is amplified according to fluctuations in the power of the monitor light. In such a beam splitter used, for example, to branch the monitor light from the signal light, it is desired to reduce polarization dependency such that a branching ratio in the beam splitter differs between P-polarized light and S-polarized light.
The beam splitter is used to branch a single light beam into two or more light beams. Conversely, the beam splitter is used also to couple two or more light beams into a single light beam. The beam splitter is configured by laminating a plurality of optical films formed of dielectric or the like on a glass substrate, and light is incident on the multilayered optical films and introduced to an optical axis with a predetermined beam incident angle set, thus functioning to reflect a part of the incident light on the multilayered optical films and transmit the remaining part. The term of beam incident angle means an angle defined between a line perpendicular to the multilayered optical films of the beam splitter and a beam of incident light. In a conventional beam splitter, the beam incident angle is set to 10.degree. or less or 45.degree., and the polarization dependency of the branching ratio such that the branching ratio is different between P-polarized light and S-polarized light on the reflection side (i.e., the difference between reflected P-polarized light and reflected S-polarized light) is about 0.5 dB. The P-polarized light means polarized light having a plane of polarization parallel to a plane of incidence on the multilayered optical films, and the S-polarized light means polarized light having a plane of polarization perpendicular to the plane of incidence.
In many cases, the beam incident angle in a short wave pass filter (SWPF) or a long wave pass filter (LWPF) used in a light amplification module or the like is set to 22.5.degree.. Accordingly, if the beam incident angle in the beam splitter used in the light amplification module or the like is set to 22.5.degree., the module can be designed with such an advantage that the optical placement of optical elements can be made simple. Further, the polarization dependency on the reflection side in the conventional beam splitter is about 0.5 dB which is a relatively large value. Accordingly, in the case where an LD (laser diode) for generating linearly polarized light is used as a light source, the branching ratio in the beam splitter changes according to a polarized condition of incident light, causing variations in characteristic of the beam splitter, which are disadvantageous in practical use.